The present invention relates to a nickel-hydrogen secondary battery. More particularly, it relates to structures of thin film type electrodes and of cells which use the electrode.
In recent years, with the spread of portable electronic equipment, development of alkaline batteries with higher capacity has been eagerly desired. In particular, nickel-hydrogen batteries, which are secondary batteries comprising a positive electrode consisting essentially of an active material containing nickel hydroxide as the main constituent and a negative electrode containing a hydrogen-absorbing alloy powder as the main constituent, are rapidly extending their use as secondary batteries with a high capacity and high reliability.
The positive electrode of alkaline batteries of prior art is described below.
The positive electrode of alkaline batteries can be broadly divided into the sintered type and the non-sintered type. The former electrode is produced, for example, by impregnating a porous sintered nickel substrate of a porosity of about 80% obtained by sintering nickel powder with a nickel salt solution, such as an aqueous nickel nitrate solution, and then immersing the impregnated substrate in an aqueous alkaline solution, thereby to form a nickel hydroxide active material in the porous sintered nickel substrate. This electrode has its limits in attaining high capacity because, since it is difficult to increase the porosity of the substrate further, the amount of the active material loaded cannot be increased further.
The latter non-sintered type positive electrode comprises, for example, as disclosed in JP-A-50-36935, a three-dimensionally continuous spongy porous substrate of a porosity of 95% or more formed essentially of nickel metal and nickel hydroxide of the active material filled in the substrate. This electrode is in wide use at present as the positive electrode of high capacity secondary batteries. In order to attain a higher capacity in the non-sintered type positive electrode, it has been proposed to fill spherical nickel hydroxide in the spongy porous substrate. In this proposal, spherical nickel hydroxide with a particle diameter of several .mu.m to several ten .mu.m is filled into the pores of the spongy porous substrate, which have a pore size of about 200-500 .mu.m. In such a structure, the charge-discharge reaction proceeds smoothly in the nickel hydroxide which is in the vicinity of the nickel metal skeleton and in which the conductive network is maintained, but the reaction does not proceed sufficiently in the nickel hydroxide detached from the skeleton. In this non-sintered type positive electrode, therefore, in order to improve the utilization factor of nickel hydroxide filled into the substrate pores, a conductive agent is used besides nickel hydroxide of the active material thereby to connect the spherical nickel hydroxide particles electrically with one another. The conductive agents used are cobalt compounds, such as cobalt hydroxide and cobalt monoxide, metallic cobalt, metallic nickel, or the like. In this way, in the non-sintered type positive electrode, the active material can be filled in a high density, and hence a higher capacity can be attained than in sintered-type positive electrode.
Next, the negative electrode of nickel-hydrogen secondary batteries using a hydrogen absorbing alloy is described below. In general, alloys of AB.sub.5 type or AB.sub.2 are used as the negative electrode active material. AB.sub.5 alloys are alloys of LaNi.sub.5 type in which, usually, the moiety of La has been replaced by Mm (Misch metal) and the moiety of Ni has been replaced by such metals as Co, Mn, Al, Fe and Cr. Misch metals are alloys of lantanide type based on La.
Such negative electrodes are produced by a method which comprises pulverizing an alloy to a predetermined particle diameter, followed by classification, mixing the alloy powder with a conductive agent and a binder into a form of paste, and then coating the paste on a nickel-plated perforated metal plate, followed by drying and pressing to obtain an electrode plate.
In the non-sintered type positive electrode of the above-mentioned structure, however, the addition of a conductive agent is necessary and, moreover, dense filling of spherical nickel hydroxide into the three-dimensional cage-like space of the spongy porous substrate is difficult to attain, and resultantly the capacity density of the positive electrode plate is about 650 mAh/cc. Furthermore, since the substrate itself has a nickel skeleton with a thickness of about 30 .mu.m, when the electrode is used, for example, to construct a cylindrical nickel-hydrogen secondary battery, sometimes broken parts are developed in the skeleton by the bending force applied in winding the electrode plate spirally, and the tip of the broken skeleton pierces through the separator to cause short-circuit with the negative electrode.